The invention generally relates to polymers employed in carbon black- and/or silica-filled vulcanizable elastomeric compositions.
When producing polymers for use in rubber articles, such as tires, power belts, and the like, it is desirable that these polymers are easily processable during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (Tg) and vinyl content. It is also desirable that reinforcing fillers, such as carbon black and/or silica, be well dispersed throughout the rubber in order to improve various physical properties, such as the compound Mooney viscosity, modulus, tangent delta (tan xcex4), and the like. Rubber articles, especially tires, produced from vulcanized elastomers exhibiting these improved properties, will have reduced hysteresis resulting in an increase in rebound, better rolling resistance, snow and ice traction, wet traction, and improved fuel economy for vehicles equipped with such tires.
With the increasing use of silica as a reinforcing filler for rubber, filler dispersion in rubber stocks has become a major concern. Because polar silanol groups on the surface of silica particles tend to self-associate, reagglomeration of silica particles occurs after compounding, leading to poor silica dispersion, a high compound viscosity and a shorter scorch time. One approach to achieving better dispersion of silica and/or carbon black during compounding involves termination of elastomeric polymers, such as diene rubbers, with a tin-containing coupling agent, such as tin tetrachloride, an organo-tin halide, a dialkyldioxastannylane compound, and the like, resulting in an increase in the Mooney viscosity of the gum polymer, which is desirable for better initial processability of the polymer. During compounding of the tin-functionalized polymers, the polymer carbon-tin bonds are cleaved, resulting in lower molecular weight fragments and, concomitantly, a lowered viscosity which allows better dispersability of the filler during compounding. Such an approach is described, for example, in U.S. Pat. No. 6,252,007.
Another approach to improving dispersion of silica and/or carbon black filler, involves modification of polymer chains with functional end groups that interact with or shield the surface hydroxyl groups on the silica filler. In particular, alkoxysilane-terminated polymers are well known and their preparation is disclosed in U.S. Pat. Nos. 3,244,664 and 4,185,042. Alkoxysilane-terminated polymers have a carbon-silicon bond and at least one terminal O-R group that reacts with the silica surface, forming an Sixe2x80x94Oxe2x80x94Si linkage.
Yet another approach to improving silica and/or carbon black dispersion involves the use of polymeric alkoxide compounds that are coupled through alkoxysilane linkages, and which are susceptible to hydrolysis under compounding conditions, resulting in decreased Mooney viscosity and improved filler dispersion. This approach is disclosed in our co-owned, copending U.S. patent application Ser. No. 09/570,578.
Although the above methods of improving filler dispersion and decreasing hysteresis of the resulting rubber compound have been shown to be successful, the need continues for alternative methods for improving the physical properties of rubber compounds, especially those used for tire treads.
The invention provides chain-coupled polymeric sulfide compounds that are especially useful as high molecular weight polymers in vulcanizable elastomeric compositions comprising silica, carbon black, or mixtures of silica and carbon black as reinforcing fillers. These compounds are particularly suitable for rubber making because of their processability, i.e., they have an initial high molecular weight (high viscosity) at synthesis for ease of handling prior to compounding, and they are extremely resistant to any increase in viscosity due to the presence of moisture during desolventization or storage prior to compounding. During compounding, the viscosity of the polymers decreases as polymer chains become decoupled, to provide a reduced viscosity and improved interaction with the reinforcing filler for better filler dispersion. Moreover, the viscosity of the resulting compound provides for good processability during extrusion or molding of the compound at the tire plant.
The use of the invention chain-coupled polymeric sulfide compounds in vulcanized elastomeric compounds for tire treads provides reduced hysteresis, resulting in an increase in rebound, better rolling resistance, snow and ice traction, and wet traction.
In particular, the polymeric sulfide compounds of the invention have the formula
(PS)nM+zPxe2x80x2zxe2x88x92n 
where P is a polymer chain; S is sulfur; Pxe2x80x2 is another polymer chain P or is an xe2x80x9cRxe2x80x9d group selected from the group consisting of alkyl groups having one to about 30 carbon atoms, aromatic groups having about 6 to about 20 carbon atoms, and cycloalkyl groups having about 5 to about 20 carbon atoms; M is a metal atom or a nonmetal atom, having an oxidation state xe2x80x9czxe2x80x9d of greater than one, wherein the nonmetal atom is selected from the group consisting of atoms of phosphorus, boron, nitrogen and sulfur; and n is an integer having a value of from 1 to z. Preferably the metal atom is selected from the group consisting of atoms of silicon, tin, titanium aluminum, arsenic, copper, calcium, zinc, lead, mercury and cadmium. More preferably, the metal atom is silicon.
In one embodiment of the invention, the polymer chain is selected from the group consisting of homopolymers of conjugated diene monomers, and copolymers and terpolymers of the conjugated diene monomers with monovinyl aromatic monomers and trienes. Preferably, the polymer chain is selected from the group consisting of polyisoprene, polystyrene, polybutadiene, butadiene-isoprene copolymer, butadiene-isoprene-styrene terpolymer, isoprene-styrene copolymer, and styrene-butadiene copolymer.
In another embodiment of the invention, one or more of these polymeric sulfide compounds are employed as the elastomeric component in a sulfur vulcanizable elastomeric composition including a reinforcing filler selected from the group consisting of silica, carbon black, and mixtures thereof, and a cure agent. The invention further provides a pneumatic tire having at least one component produced from the vulcanizable elastomeric composition.
The invention further provides a method for making the polymeric sulfide compound and a method for improving the mixing efficiency during compounding of an elastomer with a reinforcing filler by the use of the polymeric sulfide compound described above as the elastomeric component.